1. Field of the Invention
The present invention relates generally to a solid-state image pick-up device. More specifically, the invention relates to a solid-state image pick-up device comprising a charge-coupled device (CCD) of the so-called vertical overflow drain-type (V-OFD type) which drains excess electrical charge through its substrate. The invention also relates to a method for producing a solid-state image pick-up device comprising a V-OFD CCD.
2. Description of the Background Art
CCD-based solid-state image pick-up devices have recently been developed and used. In general, CCD-based solid-state image pick-up devices comprise a plurality of photosensor elements arranged in rows and columns on a semiconductor substrate. Each solid-state image pick-up device also has a plurality of vertical shift registers and a plurality of horizontal shift registers. The vertical shift registers having a CCD configuration are disposed to one side of each column of the photosensor elements and have transfer sections respectively corresponding to the adjacent photosensor elements for transferring electrical charges from the photosensor elements to the horizontal shift registers. The charge in the horizontal shift register is output through an output circuit as image data indicative of the intensity of received light along a single horizontal line.
In this type of solid-state image pick-up device, minority carriers generated at each photosensor element according to the intensity of the light received are transferred to the corresponding transfer section of the shift register corresponding to that vertical column. In each shift register, the minority carriers are transferred to the adjacent transfer section in sequence toward the horizontal shift register, so that pixel image signals according to the intensity of the received light at each photosensor elements are output through an output terminal of an output circuit of the horizontal shift register for each horizontal line in sequence, as described above.
When the photosensor element is exposed to light of excessive intensity, an excess of carriers are generated. The carriers are conducted without restraint from the photosensor elements to the shift register. This results in "blooming" on the picked-up image. In order to avoid the blooming effect, it would be possible to consider the technique of providing an overflow drain region in the vicinity of each photosensor element. In this case, however, high- density lay-out and miniaturization of the pixel elements would be made almost impossible by the area occupied by the drain regions. To resolve the aforementioned problems, a V-OFD type solid-state image pick-up device can be employed. V-OFD type solid-state image pick-up devices drain excess carriers into or through the substrate without the need for overflow drain regions in the circuit plane.
In the proposed V-OFD type solid-state image pick-up device, a potential barrier of a predetermined height is produced with respect to the carrier at a predetermined depth from the major plane of the substrate due to the presence of a well layer. The leakage of carriers into the substrate along the vertical axis can be thus limited or controlled. However, in cases where intense light is applied to the photosensor elements, thereby generating a great number of carriers, the excess carriers overflow down to the level of the barrier and then flow out through the substrate. This prevents the excess carriers from passing into the vertical shift register.
However, when such a V-OFD type solid-state image pick-up device is made from an n-type silicon monocrystal substrate obtained from an n-type silicon monocrystal body grown by the Czochralski technique (hereafter referred to as "the CZ method"), a fixed, striped pattern of noise tends to be produced. Furthermore, white dots tend to appear due to faults formed by crystal dislocations and so forth.
The fixed, striped noise pattern is due to non-uniform distribution of n-type dopant impurities, such as phosphorus, necessary for growing n-type silicon monocrystal bodies. This non-uniformity of concentration can be as much as 5%. The pitch of this concentration of non-uniformity can be as long as 60 to 400 .mu.m. This striped concentration non-uniformity is called "striation", and is caused by fluctuations in the environment or conditions surrounding the solid-liquid interface as the crystal is grown. The main factors are believed to be changes in the amount of boron (B) and oxygen (O) drawn from the inner periphery of the quartz crucible accommodating the liquid melt, due to fluctuations in crystal growth rate or to convection within the silicon melt due to temperature variations or segregation of n-type dopant from the melt during crystal growth or solidification.
The oxygen in the Si substrate is activated as a donor after heat treatment at 450.degree. C. or higher, whereupon it has the effect of suppressing increases in the number of dislocations due to heat treatment of the Si substrate; that is, it acts as a "getter" of dislocation nuclei.
In addition, recently, a method in which crystal growth is performed in a magnetic field has been proposed in conjunction with the development of crystal growth techniques disclosed in the Japanese Patent Second (examined) Publication (Tokko) Showa No. 58-50951. The modified CZ method disclosed in the above-identified Japanese Patent Second Publication will be hereafter referred to as the "MCZ method". According to the MCZ method, convection is suppressed, enabling crystal growth under stable conditions, and additionally the amount of oxygen (O) or boron (B) drawn from the crucible can stably and easily be controlled.
However, even in this MCZ method, the amount of n-type dopant in the melt appearing in the actually-grown crystal is difficult to hold to a fixed concentration due to the effect of segregation. In addition, the concentration of n-type impurity dopant in the melt may change between the start of the crystal growth and the end of the crystal growth, so that there is a gradient in concentration of n-type impurities between the top of the crystal corresponding to the start of growth and the bottom of the crystal corresponding to the end of the growth.